Help w/blowthrough on sheet metal

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Hi Folks,Newbie here - working with a Campbell Hausfeld 110V MIG, advertised 30-85A (model 105 if it makes a difference). My experience is amateurish at best, but I think I at least have the terminology and techniques down. I'm practicing lap welding some sheet metal - 22-24g if I had to guess, to reflect that found in a mid-'90s engine bay. My primary goal is to stitch weld the uni-body, as this will eventually be a purpose-build track car. The problem is I can't seem to get the settings dialed in just right. Of particular head-ache is where some thinner sheet metal mates to the "frame" - which is thicker. At the lowest setting it prevents burn-through, but not enough penetration or it doesn't adhere to the thicker metal at all. Everything is down to metal, but some of those joints still have some seam sealer between them, nearly impossible to get out w/out tearing it apart.Most of these places are inaccessible to get a backer bar on (especially against the frame rails). Can anyone provide any suggestions?Respectfully,James
Reply:From you description of the machine, I am guessing it is a flux-core mig?  Flux core can weld thicker stock than mig+gas.  Therefore, when trying to use it for thin sheet metal, it's not really the best thing, but it can be done.  First, make sure you are using .030 and not .035 wire.  Use the smaller wire for thin stuff and the larger wire for the thicker stuff.  Second, keep your arc length short and keep the end of the wire in your weld pool as much as possible.  Lean the gun back and try not to shoot it straight down.  Also, edges can give you fits.  What I do is point the gub towards the thicker stock, use a weave or circle pattern, keeping the gun on the thicker side the most, then whipping it over to grab the edge of the sheet metal.  (hope that made sense)   Practice and you'll get it.  Watch for warpage too.
Reply:If you have organic material (sealer) between the two pieces, heating it while trying to weld will cause it to turn to gas which will blow the molten metal around it away. The same is true of paint. With thick metal, you might get away with it even though the weld metal composition might be affected, but with thin material, that's not likely.
Reply:Originally Posted by JC'sWeldingFrom you description of the machine, I am guessing it is a flux-core mig?
Reply:Can you show us a picture of what you are welding?You have to keep your substrates extremely clean and bare metal.Not familiar with this mig, but you may not have as much latitude as you would like in setting feed. Typically with sheet metal you would do start stops to keep to much heat from building up, and allow plenty of time to weld peices because of warpage.Your best bet may be to get some chits of 18 gauge mild steel to practice with. Getting into the unibody you have to grind off any galvanization to welded areas.If you are building for yourself you also have to remember that any welding on unibody must mimic factory spot weld numbers keeping crushzones and flexibility intact, and also learn some normalization of spotwelds, I often speak about.Are you sure you want lap welds?A torch can meld any undercoatings, remove the rest with scotchbrite.
Reply:Hey James, The guys that have posted to your question are all correct. What you're describing is a lack of preparation. 60% to 90% of any welding job is just that. The results, (good or bad) are directly related to this. Yup, it sucks to do all the nasty stuff, but it comes with the turf. Imagine this thing comming apart at ?MPH because someone tried to weld a grunge sandwich instead of clean bare metal, who is at risk? Since you're learning, you owe it to yourself to find out what the funamentals are and stick to them. Check out the "scarey" threads by Bob Warner and a few others. Hang in there with it and let us see some pics of this thing. Good luck and keep us posted on the progress! maniak
Reply:Hi Folks,Thanks for the feedback - I'll post some picture following this, but let me address some questions first.I have indeed been skipping around to avoid any heat warping. This is partly why they aren't all in line/ as neat as I'd like, but function>form for now.Second, I should insert a disclaimer that this is a relatively low-speed track car (HPDE, maybe some weekend auto-x), not intended for the street or strip. The governing sanctions allow for re-inforcement of structural members, so I figured why not (since i'm sanding down to repaint, and I had some rust patches to fill).The purpose of the 'spot' welding is to get the car as stiff as possible - and yes crumple zones will indeed suffer.Anyway, take a look, tell me what you think. You'll see I really botched the one shock tower in one place. This was my attempt at running a small bead of about 3/4". I didn't give the area enough time to cool down I guess.On the other frame rail pictures (of slightly thicker metal) - you can see that i'm having trouble getting penetration into the backing metal.V/R,James
Reply:Prep:Apron to rad support joint - one of the better (looking at least) welds.Lack of penetration - strangely it went into the c-channel ('rail'), but not that 1" strip behind it.Spots seem OK in places, but a horrible job at a bead. I seem OK on a nice shiny flat piece on the bench, so I don't know what happened here. I'm using a A/D helmet BTW, and I've found that it really does help me.I took a leap:...and this is what I got:Last edited by slipnfall; 05-19-2009 at 10:38 PM.Reason: forgot some pictures
Reply:Post your burn throughs..looks like you may need to get more comfortable and perhaps a better sight of your welds and focus the torch from the lower sheetmetal and you may be ok, where you cleaned it well your welds improved.Good Luck
Reply:I agree, looks like you can not see what you are working on.  If you are right handed, use the gun in your right hand, pulling across while observing from the left side.  It's easier then pulling towards you and trying to look over and behind the gun.  Take a look at your spots and beads, few of them are in the right place,  and your beads miss the line all together.  Maybe it was a bad working angle?
Reply:Originally Posted by JC'sWelding...If you are right handed, use the gun in your right hand, pulling across while observing from the left side.
Reply:You want this car to be nice?As a novice at the very least consider this -Step back and run a bunch more of practice chits on 18 gauge(horizontal flat), then a bunch of spot welds or take a pair of snips and cut up an old panel(s) to practice on before you start welding shock towers because your toe-in, castor and camber are adjusted here and this is where you may make a mistake want to cut it out and fix it and make a mess of things, and the car will never steer proper.Practice, post your pictures.It will be worth it when you get to the rust.Also I don't see the point in welding these shock towers again, if you want to stiffen things up get some hat section or box section or have a sheetmetal shop make you some to put alongside your rockers or subsections and tie your seat or cage into, depending on what is sanctioned by the rulebook.Alot of schools have night courses or extension courses in mig welding.Again at the very least check here;http://www.millerwelds.com/education...rticle150.html or their literature database.Good Luck
Reply:Originally Posted by slipnfallSo you are recommending a pull type tip movement? I am really a novice, but I thought the pull kept more heat in the pool, increasing the chance of blow-through.On the rails, the lower piece of metal was about 1.5x as thick as the sheet metal above it, so I directed the pool primarily at the lower piece (and upwards towards the thinner piece). Rather should I keep the tip at 45* relative to the 90* angle formed at the lap? Then just focus more time on the heavier piece (by weaving)?My problem spot was mainly the shock tower. Here the materials are both roughly the same thickness - do I still keep a 45* angle to the lap?Thanks again!
Reply:Originally Posted by pistolnoonAlot of schools have night courses or extension courses in mig welding.Again at the very least check here;http://www.millerwelds.com/education...rticle150.html or their literature database.
Reply:re: taking a classDo it.  Yes, you'll most likely find out that nice/nicer machines can make things easier/nicer.  But it still all comes down to the person doing the weld.Next, thin sheetmetal is hard to do unless you can dial things down pretty well.Thick-to-thin means keep the puddle mostly on the thick part and then 'wash' onto the thinner part just enough to melt into that.Practice, practice, and practice some more before you weld on your car.  The best laid schemes ... Gang oft agley ...
Reply:Your welds are improving, I suspect because your limitations are from your welder only having a few taps, it may be all you need for sheetmetal until you can afford a better machine, if you can't perhaps you can get a gas kit and small tank of mixed gas.I remenber when pocket migs came out about 22 years ago you used to be able to get a small propane sized bottle of mixed gas(argon+co2) probably not worth its cost now.Keep at the horizontal beads, I'll try locate my old panels from tech school 25 years ago, but to do sheet metal work you are going to need -an autobody pick hammer, universal dolly(for backing and pushing out dents, normalizing spot welds),body file and possibly slappping file, small grinder with sanding discs, get this stuff and the next thing you know you will put putting in your notice at work to do it fulltime.I would keep at the horizontal welds and a few spot welds to get good with your machine, don't think you have a hole punch so maybe drill some holes in sheetmetal with small bit 1/8 then bore out to 9/16 or 3/8 and clamp them together, keep nozzle perfectly upright for spotwelds, don't spend much time filling hole.Keep practicing.